Restoration of the food web of an alpine lake following fish stocking

Trout stocking in the mid-1960s eliminated the calanoid copepod Hesperodiaptomus arcticus and other largebodied crustaceans such as Gammarus lacustris, Daphnia middendorffiana, and Daphnia pulex from many alpine lakes in the Rocky Mountain Parks of Canada. H. arcticus frequently dominates the plankton communities of fishless lakes, preying on rotifers and nauplius larvae. Following the extirpation of H. arcticus, rotifers and small-bodied cyclopoid copepods dominate the zooplankton assemblages of alpine lakes. We studied the zooplankton community of Snowflake Lake, Banff National Park, from 1966 to 1995. H. arcticus was eliminated following stocking of the lake with trout in the 1960s. It failed to become reestablished after the disappearance of the fish population in the mid-1980s. Several species of rotifers and small-bodied crustaceans, species originally rare or absent from the plankton, became abundant following fish stocking and remained so after the fish population declined. In 1992, we reintroduced H. arcticus to Snowflake Lake. The H. arcticus population grew exponentially for 4 yr, but had not reached stable densities typical of unmanipulated alpine lakes by 1995. By 1994, however, even the small population of Hesperodiaptomus was beginning to suppress populations of rotifers, copepod nauplii, and large diatoms. Because H. arcticus is omnivorous, a simple model of cascading trophic interactions did not predict the outcome of trophic manipulations in this alpine lake. In the early years of the twentieth century, many naturally fishless alpine lakes in the Rocky Mountains of western North America were stocked with non-native species of fish (Donald 1987; Bahls 1992). In the National Parks of the Canadian Rockies, fishless alpine lakes were often stocked in an attempt to attract anglers to the parks. For example, of an estimated 486 lakes in Banff National Park, 119 have been stocked at least once with fish. Most of the stocked lakes (84%) were originally fishless (Schindler and Pacas 1996). Many of the introduced fishes were not native to the 1 Corresponding author.